Project summary: The mammalian heart has a limited regenerative potential due to the low proliferation rate of adult cardiomyocytes. However, a narrow regenerative window has been identified in the neonatal murine heart, wherein mice are able to successfully regenerate their hearts both at the tissue and functional level following injury. In a cross-species transcriptomic screen in the Lee laboratory, inflammatory response genes including complement receptors were upregulated in a conserved manner in the regenerating heart of three model organisms ? axolotl, mouse and zebrafish. The most-upregulated inflammatory response genes include G-protein coupled receptors (GPCR) for complement proteins, complement 5a receptor (C5aR1) and complement 3a receptor (C3aR). The complement system is part of the innate immune system and enhances the ability of antibodies and macrophages to clear foreign material. Activation of an early immune response is a shared feature observed in the regenerating heart of several model organisms, consistent with the upregulation of C5aR1 and C3aR observed. C5aR1 is a GPCR that functions as a complement receptor for C5a, generated by proteolytic cleavage of complement component 5. Pharmacologic inhibition of C5aR1 after apical resection results in an impaired cardiomyocyte proliferative response in axolotl, mouse and zebrafish. I propose to investigate the role of complement activation in murine cardiac regeneration. Furthermore, I will elucidate the molecular mechanisms that initiate effective repair and cardiomyocyte proliferation in the mammalian heart following injury. I will utilize C5aR1 deletion models in zebrafish and mouse for my studies. I will examine downstream signaling components of the C5aR1 pathway that initiate cardiomyocyte proliferation following injury. To understand the cellular basis of C5aR1 signaling, I will employ a cardiomyocyte-specific deletion model of C5aR1 in mice. Furthermore, I propose to assess cardiomyocyte proliferation in C5aR1 wild-type and knock-out mice to understand the sequence of molecular events in the injured heart that initiate cardiomyocyte proliferation and effective regeneration. These studies will likely define a mechanistic pathway of early events critical for the initiation of cardiomyocyte proliferation in the myocardium.